1. Field of the Invention
The present invention relates generally to methods and burner apparatus for combusting fuel-air mixtures, and more particularly, to methods and burner apparatus for combusting fuel and air while inhibiting the formation of nitrogen oxides.
2. Description of the Prior Art
A variety of methods and burner apparatus for combusting fuel and air mixtures have been developed and utilized heretofore. Such burner apparatus are used in a great variety of applications where fuel is combusted to provide heat for a particular purpose, e.g., heating process streams, generating steam, drying materials, etc. The burning of fuels, however, can result in the formation of nitrogen oxides (NO.sub.x) which when released to the atmosphere constitute pollutants. As a result, environmental emission standards have been imposed by various governmental authorities and agencies which require the inhibition of the formation of nitrogen oxides during fuel-air combustion.
Various methods and burner apparatus for combusting fuel-air mixtures while suppressing the formation of nitrogen oxides have been developed. For example, U.S. Pat. No. 4,004,875 issued Jan. 25, 1977, is directed to a low NO.sub.x burner wherein the fuel is first burned in a zone in which there is less than a stoichiometric concentration of air thereby producing a reducing environment that suppresses NO.sub.x formation with the deficiency in air being made up in a subsequent burning zone.
Fuel staging has also been employed for suppressing NO.sub.x formation. That is, a portion of the fuel is burned in a first zone with air being supplied at a rate in excess of the stoichiometric rate required with the remaining fuel being burned in a second zone. The presence of excess air in the first zone lowers the temperature of the combustion reaction and suppresses NO.sub.x formation. The fuel in the second zone reacts with the excess oxygen resulting from the combustion in the first zone and is diluted with surrounding combustion gases which lowers the combustion reaction temperature and suppresses the formation of NO.sub.x in the second zone. A multi-stage combustion method of this type is described in U.S. Pat. No. 4,395,223 issued July 26, 1983.
While methods and burner apparatus utilizing staged combustion have been successful in reducing NO.sub.x emissions heretofore, the methods have required elaborate burner apparatus to carry out, i.e., apparatus including a plurality of fuel nozzles and/or complex air or recycle gas distribution systems making the apparatus expensive to install and operate.
By the present invention improved methods and burner apparatus for combusting fuel-air mixtures while inhibiting the formation of nitrogen oxides are provided which are simple and inexpensive as compared to prior art methods and apparatus.